The present invention relates in general to vapor delivery systems for deposition processes, and in particular to systems and methods for reliably delivering solid precursors to a deposition chamber.
Chemical vapor deposition (CVD) is a common process used in the manufacturing of films, coatings, and semiconductor devices. In a CVD process, a layer is formed on a substrate such as a semiconductor wafer by the reaction of vapor phase chemicals on or near the surface of the substrate. CVD processing is highly desirable in many applications due to it's relatively fast processing times and ability to form highly conformal layers on irregular shaped surfaces including deep contact openings.
CVD processes typically deliver one or more gaseous reactants to the surface of substrates positioned within a deposition chamber under temperature and pressure conditions favorable to the desired chemical reactions. As such, the types of layers that can be formed on a substrate using CVD techniques is limited by the types of reactants or precursors that can be delivered to the surface of the substrate.
Liquid precursors are commonly used in CVD processes due to the ease of their delivery to the deposition chamber. In typical liquid precursor systems, the liquid precursor is placed in a bubbler and heated sufficiently to transform the precursor to the vapor phase. A carrier gas typically either travels through the liquid precursor or passes over the bubbler at a controlled rate thus saturating the carrier gas with the precursor. The carrier gas then carries the liquid precursor to the surface of the substrate. Liquid precursors are commonly employed in CVD processes because the amount of liquid precursor can be precisely and consistently controlled.
The techniques developed for the delivery of liquid precursors cannot be used to reliably deliver solid precursors however. It is difficult to vaporize a solid precursor at a controlled rate such that reproducible flows are achieved. As a solid precursor sublimates, the shape and morphology of the remaining solid precursor changes. The changing volume of the solid precursor results in a continuously changing rate of vaporization. The changing rate of vaporization is notable particularly in thermally sensitive compounds. Additionally, an oversupply of vaporized solid precursor can result in condensation of the vapor back into a solid thus clogging vapor delivery lines and other monitoring equipment. Further, the use of a carrier gas is substantially ineffective as a means to implement rapid changes to the flow of the solid precursors.
Despite the difficulties in delivering solid precursors in CVD processes, there are many desirable precursor materials including for example, organometallic precursors, that are readily available in solid form. Further, many desirable precursor materials including organic and inorganic precursor materials may not be readily available in gas or liquid form. Also, solid precursors are particularly useful in the deposition of metal-based films, such as metal nitrides and metal silicides.
Therefore, there is a need in the art for a vapor delivery system for delivering solid precursors in a CVD process at a controllable rate.